Annual immunization of elderly adults and other high-risk populations with inactivated influenza virus vaccine has been recommended for the past three decades, and yet the cumulative effects of repetitive vaccination on subsequent influenza immunity are poorly understood and have not been systematically characterized in a prospective fashion. This five-year proposal will prospectively define how humoral and cellular immune responses to influenza are modulated by repetitive vaccination as well as by age, and determine whether the effects of these two host factors are independent or interrelated. The central hypothesis to be tested is that repetitive immunization with inactivated influenza virus vaccine is the primary mechanism responsible for the development of the "senescent phenotype" of immunity to influenza which is observed in older adults and which is characterized by relatively attenuated Th1-like responses and reciprocally enhanced Th2-like responses. To accomplish these aims, previously unvaccinated healthy adult volunteers aged 18-39 (young) or greater than 65 (elderly) years will be enrolled during each of five consecutive years and thereafter immunized annually with licensed influenza virus vaccine. Immune responses elicited by in vivo and in vitro restimulation with influenza antigen will be prospectively monitored on an annual basis and compared between cohorts of volunteers defined by their year of entry into the study (and hence the multiplicity of intervening vaccinations), as well as between young and elderly subjects with identical vaccination histories. In vivo responses will be assessed by measuring the quantity and immunoglobulin (Ig) isotype distribution of serum hemagglutinin-specific antibodies following vaccination. In vitro responses of virus-stimulated peripheral blood lymphocytes will be characterized by measuring virus-specific Ig class and subclass antibody production, interleukin (IL)-2, IL-4, IL-5, IL-10 and interferon gamma expression, and cytotoxic T lymphocyte activity. Age- associated differences in immunologic priming attributable to antigenic sin" responses following in vitro stimulation with prototype strains representing influenza A H1N1 viruses to which current-day young and elderly adults most likely had primary exposure. The increased susceptibility of elderly adults to serious illness following influenza virus infection is thought to be at least partially attributable to age-related changes in influenza immunity, but the mechanisms which underlie these changes are poorly understood. The information obtained from this study will enhance our understanding of influenza-specific immune senescence, and should ultimately be important for the design of new vaccines and other immunotherapies to prevent serious influenza illness in the elderly.